1. Field of the Invention
The present invention generally relates to lens focusing systems, such as the lens focusing system of a video camera, and novel digital filters particularly adapted for use with such focusing systems.
2. Description of the Prior Art
A known video camera auto-focusing system operates on the principle that the frequency components (excluding the DC component) of the video signal are at a maximum when the camera is in focus. In the known system, the frequency components of the video signal, with the exception of the DC component, of the video signal are integrated to yield an evaluation signal and the position of the lens is set to the position at which the evaluation signal reaches a maximum value, whereupon the system is in focus. The maximum value of the evaluation signal is detected by comparing sequential evaluation signals to determine whether the evaluation signal is changing from an increasing state to a decreasing state. Such a control technique is referred to as peak detecting control.
FIG. 1 illustrates an example of a conventional auto-focusing circuit of a video camera in accordance with such a system. In FIG. 1, a lens 201 is movable by a drive motor 202. An image which is transmitted through the lens 201 is converted to an electrical signal by an image pickup device 203, such as a charge coupled device (CCD). An output of the image pickup device 203 is supplied to a signal processing circuit 204 which produces a luminance signal Y therefrom and supplies the luminance signal to a detecting circuit 206 through a filter 205 which passes the frequency components of the luminance signal Y with the exception of any DC component therein. An output of the detecting circuit 206 is supplied to an A/D converter 207. An output of the A/D converter 207 is supplied to an integrating circuit 208 which is operative to integrate those portions thereof which represent a predetermined area of the image in the camera's viewfinder, thereby to provide an evaluation signal which is then supplied to a controller 210. The controller 210 controls a drive signal provided to the drive motor 202 by a driver circuit 211 to adjust the position of the lens 201 until the evaluation signal output from the integrating circuit 208 reaches a maximum. To do so, the controller 210 employs a peak detecting control technique, as described hereinbelow.
An exemplary relationship between lens position and the resulting evaluation signal is indicated by the curve provided in FIG. 2. As the lens 201 is moved from a position ln to a position ln+1, the evaluation signal Dn which is obtained at the position l.sub.n is compared with an evaluation signal D.sub.n+1 which is obtained at the subsequent position l.sub.n+1. The lens 201 is moved until the evaluation signal D.sub.n which is obtained at a prior lens position l.sub.n is larger than the evaluation signal D.sub.n+1 which is obtained at a lens position l.sub.n+1 which is achieved subsequent to the position l.sub.n.
As shown in FIG. 2, when the lens position is moved to the right, the evaluation signal increases until the lens has passed a lens position l.sub.focus at which the evaluation signal reaches a maximum value D.sub.max. When the lens has passed the lens position l.sub.focus at which the evaluation signal reaches its maximum value, the evaluation signal begins to decrease. The peak detecting control technique determines when the evaluation signals D.sub.n and D.sub.n+1 at two sequential lens positions l.sub.n and l.sub.n+1 change from an increasing state to a decreasing state while moving the lens position in a given direction. In such manner, it can be determined that the lens has reached the lens position l.sub.focus at which the evaluation signal reaches its maximum value D.sub.max, so that an in-focus position is attained.
Hitherto, a third order analog Chebyshev approximated filter has been used as the filter 205 to eliminate the DC component from the video signal in the conventional signal auto-focusing circuit. FIG. 3 illustrates evaluation signal characteristics of the conventional auto-focusing circuit using such an analog filter.
As shown in FIG. 3, in such a conventional auto-focusing circuit, the inclination of the characteristic curve of the evaluation signal is not constant. When the lens position is far from the in-focus position l.sub.focus, the inclination of the characteristic curve is nearly eliminated. Accordingly, as shown in FIG. 3, in the case where the initial position of the lens is far from the in-focus position, for example, at the location l.sub.p, the difference between the evaluation signal at the lens position l.sub.p and the evaluation signal at a lens position l.sub.p+1 which is close to the lens position l.sub.p is very small, so that it is difficult under these conditions to achieve an in-focus state.
In order to make it possible to position the lens so that the system is in-focus, even when the initial position of the lens is far from the in-focus position shown in FIG. 3, the overall gain of the circuit may be increased to increase the inclination of the evaluation signal curve when the initial location of the lens is far from the in-focus position. However, a wide dynamic range is then required which is difficult to implement.